An adjustable external-cavity laser has characteristics such as narrow spectral line width (less than 100 kHz), excellent side-mode suppression ratio (more than 45 dB), wide wavelength tunable range (more than 40 nm) and so on, and accordingly, it has been widely used as a light source at the transmitting end and as a local oscillator laser at the receiving end in the coherent wavelength-division optical communication system of 100 Gbit/s or above.
In addition, in the access network service using the technologies of wavelength-division multiplexing passive optical network (WDM-PON) and hybrid time and wavelength division multiplexing passive optical network (TWDM-PON), the multi-wavelength tunable optical transmitting module is also used.
As requirements for module size and power consumption become higher and higher, optical modules adopting an integration or hybrid integration scheme attract more attention because they can effectively reduce the power consumption and miniaturize the device and module size while ensuring the emission performance. Especially in recent years, the silicon-based optoelectronic chip technology has drawn unprecedented attention in the field of optical communication because it is compatible with the CMOS process, and it has gradually shifted from the laboratory to commercial application. As silicon has an indirect band-gap, it is very difficult to make silicon itself generate laser. Therefore, if the silicon-based optoelectronic technology is used to make a laser, a hybrid integration approach using a III-V semiconductor gain chip and a silicon-based chip is often adopted.
The Chinese Patent Application Publication No. CN103904555A discloses a tunable laser and a method of implementing the tunable laser applicable to a TWDM-PON system. A FP (Fabry-Perot) laser outputs a multi-longitudinal-mode laser signal, which enters a micro-ring filter. The micro-ring filter has a resonant frequency adjusted to match the multi-longitudinal-mode of the FP, and the optical signal of the selected longitudinal-mode is divided into two signals, one of which is input into the FP laser for injection locking mode-lock, and the other is used as an optical output signal. However, the wavelength of the optical output signal of this kind of laser is determined by the free spectral range of the FP laser, and it is difficult to accurately align to the wavelength channel value specified by the ITU-T. In addition, the multi-FP cavity effect generated by adopting the injection locking scheme also has a problematic stability of the optical power.